Nickel Sulphate, NiSO₄

Nickelous Sulphate or Nickel Sulphate, NiSO₄, has been obtained in various stages of hydration. Its solution is prepared by dissolving the hydroxide or carbonate in dilute sulphuric acid. When crystallised at 15-20° C., the heptahydrate, NiSO₄.7H₂O, is obtained as green rhombic prisms, isomorphous with the corresponding ferrous and zinc sulphates. Its density is 1.877, and specific heat 0.341.

Owing to its resemblance to ferrous sulphate or green vitriol it is frequently known as nickel vitriol. It is found in nature as the mineral moresonite. It effloresces in air, yielding the hexahydrate. At 100° C. it loses 4 molecules of water, and at 280° C. becomes anhydrous.

Its solution, evaporated at 50° to 60° C. or allowed to stand after addition of excess of dilute sulphuric acid, deposits crystals of the hexahydrate, NiSO₄.6H₂O. These are blue, tetragonal pyramids. The same hydrate is produced by prolonged exposure of the heptahydrate to air. This is an example of simple efflorescence, and is not dependent upon sunlight as was once supposed. If the crystals of heptahydrate occupy only a small portion of the containing vessel, they lose water with the formation of the hexahydrate, whether they are exposed to light or not. On the other hand, they are not changed, even by strong light, if the surrounding air is saturated with water vapour, or if the crystals are in contact with filter-paper moistened with turpentine, or if they completely fill the vessel containing them. The hexahydrate is also produced by prolonged exposure of the anhydrous salt to moist air (vide infra). The hexahydrate may further be obtained by leaving the heptahydrate for some hours in contact with its saturated solution at 32° to 53° C. The salt is dimorphous. At temperatures above 54° C. both the heptahydrate and the blue hexahydrate are rapidly converted into green monoclinic crystals containing 6 molecules of water.

The tetrahydrate, NiSO₄.4H₂O, has been prepared by crystallisation from a solution of the heptahydrate in sulphuric acid of density 1.4.

The dihydrate, NiSO₄.2H₂O, is formed from solutions by evaporation at temperatures above 118° C. It may also be obtained by the action of concentrated sulphuric acid upon the hepta- or hexa-hydrate. On heating the hexahydrate to 100° C., 5 molecules of wafer are expelled, the monohydrate, NiSO₄.H₂O, being produced. The same hydrate results when concentrated sulphuric acid is added to a solution of nickel sulphate, being precipitated from solution, and when a twice- normal solution of nickel sulphate is heated to 100° C. under a pressure of 100 atmospheres of air or nitrogen. At 280° C. the hydrated salts yield anhydrous nickel sulphate, which is yellow in colour. It absorbs moisture slowly from the air, yielding ultimately the hexahydrate, and in contact with water rehydrates itself, passing into solution if sufficient water is present. Density 3.643, specific heat 0.216. Heated to redness in air it yields nickel monoxide. Heated in a tube open at both ends it begins to decompose at 708° C. to the monoxide. Carbon reduces it to the free metal. A fifth-normal solution of nickel sulphate deposits metallic nickel at 186° C. under a pressure of 100 atmospheres of hydrogen.

It readily absorbs ammonia gas, yielding the hexammoniate, NiSO₄.6NH₃. If the anhydrous salt is dissolved in concentrated aqueous ammonia, however, a dihydrated tetrammoniate is produced, namely, NiSO₄.4NH₃.2H₂O, which separates out in dark blue tetragonal prisms which are unstable in moist air.

The solubility of nickel sulphate in water at various temperatures is as follows:

Temperature ° C.	0	9	22.6	30	34	32.3	44.7	53.0	54.5	70	99
Grams NiSO4 in 100 water	27.22	31.55	37.90	42.46	45.5	43.57	48.05	52.34	52.50	59.44	76.71
Solid phase	NiSO4.7H2O					NiSO4.6H2O blue			NiSO4.6H2O green

The transition points are as follow:

Heptahydrate ⇔ Blue hexahydrate + saturated solution [31.5° C.]
Blue hexahydrate ⇔ Green hexahydrate [53.3° C.]
Green hexahydrate ⇔ Dihydrate + saturated solution [uncertain]

Several Basic Nickel Sulphates have been described, but these are of less interest and importance.

Nickel sulphate yields numerous double salts. Of these the most important is Nickel Ammonium Sulphate, (NH₄)₂SO₄.NiSO₄.6H₂O, which is largely used in electroplating. It is readily prepared by adding the requisite quantity of ammonium sulphate to a concentrated solution of nickel sulphate in dilute sulphuric acid. It crystallises in bluish green monoclinic prisms. These are less soluble than the corresponding cobalt salt.

Temperature ° C.	3.5	10	16	20	30	40	50	68	85
Grams of salt in 100 grams water	1.8	3.2	5.8	5.9	8.3	11.5	14.4	18.8	28.6

The corresponding Potassium, Rubidium, and Caesium salts have been prepared, and their crystallographic constants determined.

Anhydrous Nickel Ammonium Sulphate, 3NiSO₄.2(NH₄)₂SO₄, has been prepared by fusing together nickel sulphate, oxide, hydroxide, or carbonate and excess of ammonium sulphate. It combines with water evolving heat. When ignited, anhydrous nickel sulphate remains behind.